The broad objective of our work is to understand the neural regulation of cardiac conduction and excitation under a variety of natural and experimental conditions. This proposal will build upon certain of my previous observations that have been unique to our laboratory in this area, and that seem to be of fundamental importance to a detailed understanding of the underlying basic physiology. The specific objectives of the proposed work are: 1) to investigate in detail the time dependent nature of the SA and AV nodal responses to activity of each autonomic nervous system division alone, as well as the vagal-sympathetic interaction, for both carefully timed single, and continuous trains of stimuli; 2) to study the determinants of AV nodal vagal effect curve configurations, including the role of vagal-sympathetic interactions, the effects of cardiac glycosides and antiarrhythmic drugs, and 3) to investigate more fully the nonlinear interaction between dynamically changing heart periods and neural activity on AV conduction, for both vagal and sympathetic activity alone and in terms of a vagal-sympathetic interaction, and when cardiac pacemaking activity is initiated by an ectopic pacemaker; and 4) to study the neural genesis of certain cardiac arrhythmias, especially the Wenckebach rhythm, first with carefully timed efferent neural stimulation alone, and then by means of the more physiologic mechanism via the barorecptor reflex.